Micellar lipid-apolipoprotein A-I (apo A-I) complexes were produced by adding human apo A-I to lipid dispersions containing sodium cholate. The lipid mixtures included dimyristoyl phosphatidylcholine (DMPC), dipalmitoyl phosphatidylcholine (DPPC), and egg phosphatidylcholine (egg-PC). The formation of complexes was examined at increasing cholesterol contents with DPPC, and at 20 mole % cholesterol contents for DMPC, DPPC, and egg-PC.The average molar ratio of PC to apo A-I in the complexes depended on the initial lipid to protein ratio; however, a stable minimum size complex was present in all the reaction mixtures. The yields of micellar complexes were highest at incubation molar ratios (PC:apo A-I) near the stoichiometry of the isolated, minimum sized complex and at incubation temperatures approaching the transition temperature of the corresponding PC. For the different micellar complexes, the PC:apo A-I molar ratios decreased as the average fatty acid chain length and/or unsaturation increased, i.e. 160:1 for DMPC, 125:1 for DPPC, and 95:1 for egg-PC. The complexes prepared with the different lipids had similar physical properties including size, shape, buoyant densities and protein secondary structure; they appeared as lamellar, discoidal particles when negatively stained and viewed by electron microscopy.Micellar complexes containing the various phosphatidylcholines and 20 mole % cholesterol were used as substrate particles for human lecithin cholesterol acyltransferase (LCAT), purified 10,000-fold. The initial enzymatic reaction velocities decreased in the order of egg-PC, DPPC, ad DMPC. When compared to commonly used substrates consisting of small unilamellar vesicles of egg-PC and cholesterol in the presence of apo A-I, the micellar complexes had 4- to 5-fold higher initial velocities and 3-fold greater capacities for cholesterol esters.Micellar complexes, labeled with 5-dimethylamino-naphthalene-1-sulfonyl fluorescent groups in the apoprotein, were reacted with LCAT and reisolated. After reaction, the complexes had a shorter rotational relaxation time (290 ns) and smaller Stokes radius (47 (ANGSTROM)) than the unreactecomplexes (530 ns and 57 (ANGSTROM), respectively). The stacked, discoidal particles observed on electron micrographs of micellar complexes disappeared after enzymatic reaction and were replaced by structures with spheroidal shapes.